IX] PHOTO-SYNTHESIS 95 



too strong, or if no carbon-dioxide is allowed to reach the 

 chlorophyll-corpuscles then these little starch-grains do 

 not appear. And if the light, or the carbon-dioxide, or 

 other necessary conditions are withheld too long, then the 

 chlorophyll-corpuscles turn pale and die, and the cell 

 containing them soon dies also. If the light is only 

 withheld for a day or two, however, it is found that 

 any starch already present slowly disappears ; and we 

 know that it is dissolved and turned into sugar and 

 sugar-like bodies. 



Now what is the meaning of all this ? Put briefly, it 

 means essentially that during the light of day, the carbon- 

 dioxide which passes into the mesophyll-cells from the 

 intercellular passages, is seized by the chlorophyll-cor- 

 puscles, and by means of the energy of the solar rays is 

 broken up : its carbon is retained and made to unite 

 with oxygen and hydrogen obtained from the water sup- 

 plied by the vessels, and built up into the starch, sugars 

 and such like carbonaceous substances referred to. It 

 is from these carbonaceous bodies that the plant obtains 

 its materials for forming new cellulose, oil, and other ma- 

 terials out of which new plant-substance is manufactured. 



It would take up more space than I can afford here to 

 explain what we know of the further details, but it is 

 important to understand that during this process of Photo- 

 synthesis of the carbonaceous food from carbon-dioxide 

 and water, oxygen is set free and diffuses out into the 

 intercellular spaces. 



And now we see whence the efferent vessels of the 

 venation get their supplies of sugars and such like carbon- 

 aceous bodies, and why our hypothetical traveller in the 

 intercellular labyrinth would experience such great changes 

 in the atmosphere of the tunnels according to the time of 

 day or night. 



